The battery, or more specifically, a secondary/rechargeable battery, which is chargeable and dischargeable, may have its storage capacity quickly deteriorated when over-charged and over-discharged. Therefore, based on grasping/detecting a state of charge (SOC) of the battery, the charging and discharging have to be controlled. Therefore, the SOC of the battery is typically kept around a middle range in between the full charge (100% SOC) and no charge (0% SOC), which is 50 to 60% SOC. That is, in other words, the SOC may have to be more accurately grasped/detected.
The technique disclosed in the following document Japanese Patent Laid-Open No. 2013-176195 (Patent document 1) is about an intentional over-charging of the battery for an improvement of SOC estimation accuracy, which may gradually deteriorate during the use of the battery. That is, the over-charged state of the battery is assumed/considered as 100% SOC according to the technique of the patent document 1, which is thus designated as an SOC reset charging.
The conventional technique described above dissolves the SOC calculation error by the 100% SOC charging of the battery with a night-time grid power, when a dispersion of terminal voltages among many batteries exceeds a threshold value, based on once-a-day terminal voltage checking at a certain timing of the day.
However, the SOC reset charging in such manner is initiated/triggered by a detected state of the battery, which may not necessarily be a best timing as a whole system, in term of charge timing and the like. More specifically, as an example, the charge timing may optimally be adjusted based on the price of the night-time grid power (where the price may be lower than day-time power), which is not the case in the battery-state-triggered charging technique of the patent document 1.